Optical head apparatus

ABSTRACT

To implement light quantity monitoring with high frequency responsivity and correction of astigmatic differences of a semiconductor laser with a simple configuration with fewer parts. Of the output from a semiconductor laser light source, a peripheral component is entered by a light reflection element into an anterior light monitoring photodetector formed in the vicinity of a semiconductor laser light source. Furthermore, the surface of the reflection sphere of the light reflection element is anamorphic, and thus condensed to an appropriate size on the photodetector without being focused, providing high frequency responsivity. Furthermore, the light reflection element is inclined at a predetermined angle to cancel out astigmatic differences of the optical semiconductor laser light source. In addition, the photodetector is placed so that reflected light is bent by an inclination of the light reflection element, reducing the amount of parallel displacement during adjustment of the light reflection element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical head apparatus that performsrecording or reproduction of optical information recording media.

2. Description of the Prior Art

Generally, a rewritable type optical disc must monitor the quantity ofincident light to the recording surface of the disc to secure the signalrecording quality with high accuracy. For this reason, the accuracy of asystem that monitors the light quantity using light output from theposterior end face of a laser chip used in a reproduction-only opticalhead is not high, and therefore it is necessary to monitor the lightquantity using light radiated from the anterior end face of the laserchip (hereinafter referred to as “anterior light”).

On the other hand, while optical discs are attracting attention aslarge-capacity information memories, optical head apparatuses need toattend a demand for high-speed recording or reproduction of opticaldiscs. To meet this demand, it is necessary to increase the speed ofmodulation of a semiconductor laser light source and at the same timeimprove responsivity of the above described monitoring of the anteriorlight.

A conventional optical pick up will be explained with reference to theattached drawings below. FIG. 14 shows an example of an outlinedconfiguration of a conventional optical head, apparatus. A divergingbeam 802 radiated from a semiconductor laser light source 801 passesthrough a parallel flat plate 803 placed diagonally to the optical axisand is converted to a parallel beam 805 by a collimate lens 804.

This collimated beam 805 is partially reflected by a polarized beamsplitter 806 and enters into a photodetector 809. A beam 810, the majorportion of the collimated beam 805, passes through the polarized beamsplitter 806 and is converted to a circularly polarized beam by a ¼wavelength plate 811, and then condensed into an optical disc 814through an objective lens 813 mounted on an actuator 812.

The beam reflected by the optical disc 814 passes through the objectivelens 813 and is converted by the ¼ wavelength plate 811 to a linearlypolarized beam, which is orthogonal to the polarization plane of theoutgoing radiation beam of the semiconductor laser light source 801 andentered into the polarized beam splitter 806.

Since the polarization plane of the incident beam entered into thepolarized beam splitter 806 is orthogonal to the first half of theoptical path, the incident beam is reflected by the polarized beamsplitter 806, diffracted by a hologram element 815, branched into apositive 1st-order diffracted light 817 and negative 1st-orderdiffracted light 818 with the optical axis of the incident light as anaxis of symmetry, then condensed by a detection lens 817, entered intosignal detectors 820 and 821, respectively, to detect control signalssuch as focusing and tacking, and RF signals.

On the other hand, photodetector 809 that detects light reflected by thepolarized beam splitter 806 acts as an output light quantity monitor ofthe semiconductor laser light source 801.

Here, the reason why the parallel plate 803 is placed diagonally to theoptical axis of the incident beam between the semiconductor laser lightsource 801 and collimate lens 804 will be explained. Generally, as for asemiconductor laser used for a light source of the optical headapparatus, from the standpoint of an optical characteristic, mode westof an oscillated beam of a semiconductor laser element 901 differsbetween the semiconductor composition plane (X-Z axial plane) and theplane normal thereto (Y-Z axial plane) as shown in FIG. 15.

That is, while the mode west is a point that matches a specular surface902 within the perpendicular (Y-Z axial plane), it is a point inside anactivated layer 903 of the semiconductor laser element 901, that is, apoint at a certain depth from the specular surface 902 into theresonator within the composition plane (X-Z axial plane).

Therefore, the converging point of the oscillated beam differs betweenthe composition plane (X-Z axial plane) and the plane normal thereto(Y-Z axial plane), and thus an “astigmatic difference” 904 in opticalterms is produced.

When an astigmatic difference occurs, the beam spot is distorted into aflat, vertically or horizontally oblong spot. Therefore, the beam spotspans mutually neighboring recording tracks of an optical disc, causinga problem of deteriorating a signal characteristic.

It is for this reason that in FIG. 14, the parallel plate 803 is placedinclined at a predetermined angle in the reverse direction in order tocorrect the astigmatism of the light beam radiated from thesemiconductor laser 801.

Moreover, another method proposed to correct such astigmatism of a lightbeam is canceling out the astigmatism of the light spot by inserting acylindrical lens in the same optical path of the laser beam.

BRIEF SUMMARY OF THE INVENTION Object of the Invention

The above described conventional optical head apparatus has thefollowing problems:

Generally, when recording a signal on a rewritable type optical disc, itis necessary to secure sufficient optical power on the disc, andtherefore the light utilization efficiency of the optical head must besecured.

However, the configuration of the above described conventional exampleperforms no beam shaping, and therefore abandons a portion of light inthe outer regions for reasons related to the design of the objectivelens, which means a loss of light quantity.

Furthermore, a part of the beam within the effective aperture isreflected and used by the photodetector 809 to monitor the lightquantity, which increases the loss all the more. To avoid this, loweringthe light quantity to be conducted to the light quantity monitor andincreasing the light quantity within the effective aperture willdeteriorate the S/N ratio of the monitor signal.

Moreover, increasing the speed of laser modulation requires theresponsivity of the anterior light monitor itself to be improved. Forthis reason, it is preferable to reduce the photoreception area of thephotodetector and input a condensed beam in order to improve theresponse frequency characteristic of optical detection.

However, exposing the photodetector to an excessively condensed beamwill increase the light intensity per unit area of the detector surface,increasing the carrier density on the photoreception surface of thedetector, which then becomes saturated causing the traveling speed ofcarriers to slow down. That is, condensing the beam on the detectorexcessively may cause a problem of deteriorating the response frequencycharacteristic of optical detection.

Furthermore, all the above described methods to correct the astigmatismof a light beam produced by an astigmatic difference among thesemiconductor laser elements above must provide special parts such as atransparent parallel plate and cylindrical lens separately, causing anadditional problem of unavoidably increasing the number of parts, hencecost increase.

In addition, since the photodetector for an RF signal, focusing ortracking control signals is provided apart from the photodetector forlaser light quantity monitoring, which increases the number of parts andcomplicates the optical system, making it difficult to reduce the sizeof the optical head.

The present invention has been implemented taking into account theseproblems of the conventional optical head apparatus and it is an objectof the present invention to provide an optical head apparatus with highlight utilization efficiency.

It is another object of the present invention to provide a compactoptical head apparatus.

It is still another object of the present invention to provide anoptical head apparatus with an excellent response frequencycharacteristic of optical detection.

SUMMARY OF THE INVENTION

Therefore one aspect of the present invention is an optical headapparatus, comprising:

a semiconductor laser light source;

a photodetector that receives at least one part of light from saidsemiconductor laser light source;

a light reflection element provided with a peripheral section thatreflects peripheral light of to the light from said semiconductor laserlight source and condenses it into said photodetector and a centralsection that transmits central light of the light from saidsemiconductor laser light source; and

a condenser lens that condenses the light that passes through said lightreflection element onto an optical disc,

wherein:

each surface of the central section of said light reflection element hasa flat shape; and

at least one surface of the peripheral section of said light reflectionelement has a spherical or non-spherical shape.

Therefore another aspect of the present invention is an optical headapparatus, comprising:

a semiconductor laser light source;

a photodetector that receives at least one part of light from saidsemiconductor laser light source;

a light reflection element provided with a function of reflectingperipheral light of the light from said semiconductor laser light sourceand condensing it into said photodetector and a function of transmittingthe central light of the light from said semiconductor laser lightsource; and

a condenser lens that condenses the light that passes through said lightreflection element onto an optical disc,

characterized in that said semiconductor laser light source and saidphotodetector are formed in one package.

Therefore still another aspect of the present invention is an opticalhead apparatus, comprising:

a semiconductor laser light source;

a plurality of photodetectors placed adjacent to said semiconductorlaser light source;

a reflection type hologram element provided with a peripheral sectionthat reflects and diffracts peripheral light of the light from saidsemiconductor laser light source and condenses it into one of saidplurality of photodetectors and a central section that transmits centrallight of the light from said semiconductor laser light source; and

a condenser lens that condenses the light that passes through thecentral section of said reflection type hologram element onto an opticaldisc,

wherein:

said photodetector that receives said reflected and diffracted light isplaced closer, with respect to said semiconductor laser light source, inthe direction of the major axis of an ellipse than in the direction ofthe minor axis of the ellipse of an elliptic far field pattern ofoutgoing light from said semiconductor laser light source; and

the photodetector that receives signal light from said optical disc isplaced closer, with respect to said semiconductor laser light source, inthe direction of the minor axis of the ellipse than in the direction ofthe major axis of the ellipse of an elliptic far field pattern ofoutgoing light from said semiconductor laser light source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outlined configuration of an optical head according to anembodiment of the present invention;

FIG. 2 is a drawing showing a configuration of elements used in theabove embodiment of the present invention;

FIG. 3 a schematic drawing of spot shapes of the above embodiment of thepresent invention;

FIG. 4 an outlined configuration of an optical head according to anotherembodiment of the present invention;

FIG. 5 a schematic drawing of spot shapes of the above embodiment of thepresent invention;

FIG. 6 an outlined configuration of an optical head according to anotherembodiment of the present invention;

FIG. 7 a drawing showing a configuration of elements used in the aboveembodiment of the present invention;

FIG. 8 a configuration diagram of an optical head apparatus of anotherembodiment of the present invention;

FIG. 9 a plan view of a reflection type hologram according to anotherembodiment of the present invention;

FIG. 10 a layout of a reflection type hologram element, laser lightsource and photodetector according to another embodiment of the presentinvention;

FIG. 11 a configuration diagram of an optical head according to anotherembodiment of the present invention;

FIG. 12 a layout of a reflection type hologram element, laser lightsource and photodetector according to another embodiment of the presentinvention;

FIG. 13 a layout of a reflection type hologram element and photodetectoraccording to another embodiment of the present invention;

FIG. 14 a drawing showing a conventional optical head apparatus; and

FIG. 15 a schematic drawing showing an astigmatic difference of asemiconductor laser.

(Description of Symbols)  1 Laser light source  2 Reflection typehologram element  3 Collimate lens  4 Objective lens  5 Polarizedhologram element  6 Anterior light monitoring photodetector  7 Signaldetection photodetector  8 Optical disc plane  9 Actuator  10 Reflectionhologram 101 Semiconductor laser light source 102 Divergent light 103Photodetector 104 Collimate lens 105 Parallel light 106 Polarized beamsplitter 107 Light reflection element 108 Reflected light 109 Opticalintegrated module 110 Transmission light 111 ¼ wavelength plate 112Actuator 113 Objective lens 114 Optical disc 115 Hologram element 116Detection lens 117 Positive 1st order diffracted light 118 Negative 1storder diffracted light 119 Signal detector 120 Signal detector 201 Laseroptical axis 202 Transmission plane 203 Aluminum-evaporated plane 407Light reflection element 607 Reflection type hologram element 608Reflected/diffracted light 701 Laser optical axis 702 Transmission plane703 Reflection hologram plane 801 Semiconductor laser light source 802Divergent light 803 Parallel plate 804 Collimate lens 805 Parallel light806 Polarized beam splitter 807 Reflected light 809 Optical integratedmodule 810 Transmission light 811 ¼ wavelength plate 812 Actuator 813Objective lens 814 Optical disc 815 Hologram element 816 Detection lens817 Positive 1st order light 818 Negative 1st order light 819 Signaldetector 820 Signal detector

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIG. 1 through FIG. 7, embodiments of the presentinvention will be explained below. Detailed explanations of the partsthat have the same functions as those in the conventional example willbe omitted.

FIG. 1 shows an outlined configuration of the optical head apparatus ofan embodiment of the present invention and FIG. 2 shows theconfiguration of a light reflection element, laser and photodetectorthat are used as components of optical head apparatus. A lightreflection element 107 used in the configuration in FIG. 1 is made ofglass and provided with a flat shaped transmission plane 202 at itscenter section (corresponding to the central section in the presentinvention) with a centered laser optical axis 201, and a non-sphericallyformed aluminum-evaporated plane 203 at a ring-figured area(corresponding to the peripheral section of the present invention)surrounding said transmission plane 202 as shown in FIG.2.

Among the light beam radiated from the semiconductor laser light source101 in FIG. 1, the peripheral beam component is reflected and condensedby the light reflection element 107 and condensed into a photodetector103 for anterior light monitoring formed in the vicinity of thesemiconductor laser light source 101. The semiconductor laser lightsource 101 and photodetector 103 for anterior light monitoring areintegrated into an optical integrated module 109 for the purpose ofreducing the size and weight.

On the other hand, the beam at the central section is collimated by acollimate lens 104 into a parallel beam, passes through a polarized beamsplitter 106, and is condensed by an objective lens 113 mounted on anactuator 112 onto the surface of an optical disc 114.

The beam reflected by the optical disc 114 passes through the objectivelens 113 and is converted by a ¼ wavelength plate 111 to linearlypolarized light orthogonal to the polarization plane of thesemiconductor laser outgoing radiation beam and is entered into thepolarized beam splitter 106.

Since the polarization plane of the incident beam entered into thepolarized beam splitter 106 is orthogonal to that in the case of thefirst half of the optical path, the incident beam is reflected by thepolarized beam splitter 106 and diffracted by a hologram element 115.The diffracted beam is branched into a positive 1st-order diffractedlight 117 and negative 1st-order diffracted light 118 with the opticalaxis of the incident light as an axis of symmetry, then is condensed bya detection lens 116, entered into signal detectors 119 and 120,respectively, and used for control signals such as focusing and tacking,and RF signals.

The astigmatism of the light beam radiated from the semiconductor laser101 is compensated by inclining the light reflection element 107 at apredetermined angle.

Moreover, as shown in FIG. 1, due to an inclination of the lightreflection element 107, the optical axis of the reflected light 108 isinclined with respect to the optical axis of the light from thesemiconductor laser light source. The photodetector 103 is placed in thedirection of the reflected light 108. Here, when manufacturing, it isnecessary to adjust so that the reflected light 108 enters into thephotodetector 103. For this purpose, it is necessary to adjust thecentral axis of the reflection spherical plane of the light reflectionelement 108 through parallel displacement of the central axis in thedirection indicated by an arrow in FIG. 1 as appropriate. The abovedescribed inclination of the photodetector 103 reduces the amount ofparallel displacement compared to the case without the inclination.

The ability to reduce the amount of parallel displacement for adjustmentcan reduce the area of the central flat section of the light reflectionelement 107, form a wider light reflection surface of the peripheralsection and capture more reflected light. This is because largerparallel displacement for adjustment requires the central flat sectionof the light reflection element 107 to be designed with more allowancebeforehand.

Furthermore, since it is possible to design a smaller angle ofreflection of the reflection spherical surface, the reflection plane ofthe light reflection element 107 becomes a spherical plane with smallercurvature, making it easier to shape the reflection plane.

Furthermore, if the peripheral section of the light reflection element107 is formed in an anamorphic, non-spherical shape, as shown in FIG. 2,the focus onto which light is reflected and condensed by the lightreflection element 107 differs between the x-z axial plane and the y-zaxial plane normal thereto in FIG. 2. That is, the focus of spots on thesurface of the photodetector 103 has an astigmatic difference as shownin FIG. 3. Even if defocusing occurs due to errors in mounting opticalparts, etc., the astigmatic difference increases the light intensity perunit area of the surface of the photodetector, and preventsdeterioration of frequency responsivity caused by concentration ofcarriers.

FIG. 4 shows an embodiment using a light reflection element 407 withreflected and converged light having a spherical aberration instead ofthe light reflection element 107 with reflected and converged lighthaving an astigmatic difference in the embodiment of FIG. 1. In thiscase, the spot focus on the surface of the photodetector 103 has aspherical aberration as shown in FIG. 5. With this, even if defocusingoccurs due to errors in mounting optical parts, etc., it is possible tomaintain lower light intensity per unit area of the surface of thephotodetector than spots without spherical aberration, and preventdeterioration of frequency responsivity caused by concentration ofcarriers.

FIG. 6 shows a schematic drawing of another embodiment of the presentinvention. A reflection type hologram element 607 used in theconfiguration in FIG. 6 is provided with a reflection hologram plane 703formed in a ring-figured area (corresponding to the peripheral sectionof the present invention) at the peripheral section for a laser opticalaxis 701 as a center position as shown in FIG. 7 and there is a circularlight transmission area 702 (corresponding to the central section in thepresent invention) in the vicinity of the laser optical axis 701.

Among the light beam radiated from the semiconductor laser light source101 in FIG. 6, the peripheral beam component is reflected and diffractedby the reflection hologram 607. The pitch and groove orientation of thisreflection type hologram are different depending on the incident lightand incident position of the laser beam and the reflected/diffractedbeam 608 is condensed into the photodetector 103 placed in the vicinityof the semiconductor light source 101.

Furthermore, since this reflection type hologram element 607 is formedto have an astigmatic difference as in the case of the above describedlight reflection element 107, a spot beam does not form a focus butfocal line. With this, even if defocusing occurs due to errors inmounting optical parts, etc., it is possible to maintain lower lightintensity per unit area of the surface of the photodetector than spotswithout astigmatic differences, and prevent deterioration of frequencyresponsivity caused by concentration of carriers.

Furthermore, for manufacturing adjustment, it is necessary to enter thereflected/converged light from the reflection type hologram element 607into the photodetector through parallel displacement of the reflectiontype hologram element 607 in the direction indicated by an arrow in FIG.6. In this case, as shown in FIG. 6, placing the photodetector 103 inthe direction that the light is reflected by the inclined reflectiontype hologram element 607 can reduce the amount of parallel displacementto adjust the reflection type hologram element 607.

This can reduce the angle of diffraction of the reflection type hologramelement 607 compared to the case where the reflection type hologramelement 607 is not inclined and allows the reflection type hologramelement 407 to be designed with a wider pitch, securing an advantage inrespect of hologram elaboration pitch limitations.

Here, in the embodiment in FIG. 6, it goes without saying that using alight reflection element with the reflected/diffracted light having aspherical aberration instead of the reflection hologram element 607 withthe reflected/diffracted light having an astigmatic difference will alsoobtain effects similar to those in the embodiment in FIG. 4.

FIG. 8(a) shows an outlined configuration of the optical head apparatusaccording to another embodiment of the present invention and FIG. 8(b)shows a layout of the reflection type hologram element, laser andphotodetector used as its components.

The reflection type hologram element 2 used in the configuration in FIG.8(a) consists of a reflection type hologram 10 formed in a ring-figuredarea in the outer circumference for the laser optical axis 15 as acentral position and a circular light transmission area 11 in thevicinity of the laser optical axis 15 as shown in FIG. 8(b)

Of the light beam radiated from the semiconductor laser light source 1in FIG. 8(a), the peripheral beam component 22 is reflected anddiffracted by the reflection type hologram 10. The pitch and grooveorientation of this reflection hologram are different depending on theincident light and incident position of the laser beam and thereflected/diffracted beam 23 is condensed into the photodetector 6 foranterior light monitoring placed in the vicinity of the semiconductorlight source 1.

On the other hand, among the beam radiated from the semiconductor laserlight source, the inner beam component passes through the reflectiontype hologram element 2 with restricted aperture, polarized hologramelement 5 and ¼ wavelength plate 20, and is converted to a parallel beamby a collimate lens 3, and then condensed into an optical disc 8 throughan objective lens 4 mounted on an actuator 9.

The beam reflected by the optical disc 8 is diffracted by the objectivelens 4, collimate lens 3, ¼ wavelength plate 20 and polarized hologramelement 5 and entered into a signal detection photodetector 7 formedaround the semiconductor laser light source 1 for detection of signalssuch as RF signal, focusing and tracking control signals.

With such a configuration using outer circumferential light componentthat is originally not used for anterior light monitoring, the presentembodiment can improve the light utilization efficiency of the opticalhead and integrate all the semiconductor laser light source 1, signaldetection detector 7, detector 6 for anterior light monitoring in asingle unit, thus reducing the number of parts of the optical headapparatus.

Furthermore, the reflection type hologram 10 having a condensingfunction allows light beam to be condensed into a photodetector with asmall area without another condensing means such as a lens, simplifyingand reducing the size of the optical head apparatus while securinghigh-speed responsivity of anterior light monitoring.

Here, as shown in FIG. 8(b), the semiconductor laser light source 1,anterior light monitoring photodetector 6 and signal detectionphotodetector 7 are configured as follows:

That is, for a laser outgoing radiation far field pattern 12 as shown bydotted line in FIG. 8(b), the anterior light monitoring photodetector 6is placed in the direction close to the direction 13 of the major axisof the ellipse and signal detection photodetector 7 to detect signalsfrom the optical disc is placed in the direction 14 close to thedirection of the minor axis of the ellipse.

That is, the anterior light monitoring photodetector 6 is placed closeto the direction of the major axis of the ellipse rather than the minoraxis with respect to the optical axis of the light from thesemiconductor laser light source. On the other hand, the photodetector 7for signal detection is placed close to the direction of the minor axisof the ellipse. For example, it is preferable that they be placed in thedirection of the major axis and minor axis of the ellipse, respectively.

That is, such a configuration has the following effects. The reflectionhologram 10 produces a beam with the order which diffracts toward theanterior light monitoring photodetector 6, a beam with the order whichdiffracts in its opposite side and a 0-order diffracted beam, but sinceunnecessary beams other than the beam with the order which diffractstoward the anterior light monitoring photodetector 6 travel in thedirection of the major axis of the ellipse of the far field pattern,these beams are not entered into the photodetector 7 to detect a signalfrom the optical disc as stray light.

Furthermore, since each photo detector can be placed close to the laserchip, requiring only a small angle of diffraction by the reflectionhologram 10 or polarized hologram element 5, it is possible to have alarge hologram pitch and secure sufficient allowance for hologramelaboration pitch limitations.

FIG. 9 shows a plan view of the reflection type hologram element 2according to another embodiment of the present invention which sets thespreading angle of a laser light source, the hologram area and arelative distance between the light source and hologram so that morelight in the major axis of the ellipse of the outgoing radiation farfield pattern 12 of the semiconductor laser light source is reflectedand diffracted.

Generally, a density distribution of a semiconductor laser changes dueto temperature variations more in the direction of the minor axis thanin the direction of the major axis. This change affects the linearity ofthe light quantity of anterior light monitoring and the light quantityof light passing through the reflection hologram element 2. Therefore,for a system requiring control of light quantity with very high accuracyas in the case of the present embodiment, it is preferable to use onlylight in the direction of the major axis as the light for anterior lightmonitoring. That is, the hologram 10 formation area of the reflectiontype hologram element 2 is formed more widely in the direction of themajor axis of the ellipse with respect to the center of the axis of theelliptic far field pattern of the above described semiconductor laser.

FIG. 10 shows a plan view of the reflection type hologram and thelocation of the photodetector in another embodiment of the presentinvention. In the present embodiment, the area of the reflection typehologram 10 is formed asymmetric with respect to a point centered on thelaser optical axis 15. That is, if a laser beam is reflected/diffractedby the anterior light monitoring photodetector 6, which is deviated fromthe laser optical axis 15, the hologram pitch varies sequentiallydepending on its incidence angle and position. However, since this pitchalso has elaboration limitations, the area is subject to theselimitations.

However, some directions allow a large distance from the optical axis tothe pitch elaboration limitations, and therefore it is possible toincrease the light quantity of reflected/diffracted light for anteriorlight monitoring by forming the reflection type hologram 10 up to theboundary 17 of the elaboration pitch limitations indicated by the areaasymmetric with respect to a point as shown in FIG. 10.

FIG. 11(a) shows an optical head apparatus in another embodiment of thepresent invention and FIG. 11(b) shows a plan view of a reflection typehologram element used for its configuration.

As shown in FIG. 11(b), the reflection type hologram element 2 has anoval or slotted-hole shaped light transmission area 11 at the center. InFIG. 11(a), a polarized hologram element 5 and ¼ wavelength plate 20 todiffract the reflected light from the optical disc and lead it to thephotodetector 7 for detection of signals such as focusing and trackingare mounted together with an objective lens 4 on the movable part of anobjective lens actuator 9.

Therefore, when the objective lens 4 moves in the direction orthogonalto the track in order to follow up tracking errors due to eccentricityof the optical disc, the light 25 diffracted by the polarized hologramelement 5 also moves together (solid arrow 25 → dotted line arrow 25′).

According to the reflection type hologram element 2 in FIG. 11(b), thelight transmission area 11 of the reflection hologram 10 extends widelyin this direction of movement, and therefore it is possible to implementa structure that prevents shading the signal detection light minimizingthe reduction of light quantity of reflected/diffracted light foranterior light monitoring. FIG. 11(a) and FIG. 11(b) depict trackingoperation directions with the vertical and horizontal directionsreversed.

FIG. 12 shows a part of the optical head apparatus of another embodimentof the present invention. In FIG. 12, a polarized hologram element isformed with a polarized hologram layer 26 and ¼ wavelength film 19sandwiched between two glass plates. Furthermore, a reflection typehologram 10 is formed on a glass substrate on the other side.

This allows the elements to be integrated and simplifies theconfiguration of the optical head and at the same time allows, when thereflected/diffracted light spot of the reflection type hologram 10 ispositioned on the monitoring photodetector, the signal detectionhologram to be positioned simultaneously, making it possibly to simplifyadjustment in the optical head manufacturing process.

FIG. 13 shows a reflection type hologram element 2, anterior lightmonitoring photodetector 6 and light beam diffracted by the reflectiontype hologram of the optical head apparatus according to anotherembodiment of the present invention.

As shown in FIG. 13, the condensing point of the light beams reflectedand diffracted by the reflection type hologram element 2 is defocusedbefore and after the photodetector 6 because the wavelength of the laserlight source fluctuates due to temperature variations, etc.

In order to prevent the light beam from going off the edge of theanterior light monitoring photodetector 6 due to such defocusing, it ispreferable to design so that the condensing point matches the plane ofthe photodetector at a midpoint 32 between a focus point 31 at theminimum temperature in the operating temperature range of the opticalhead and a focus point 30 at the maximum temperature. This reducesvariations of the monitoring light quantity even with variations in thelaser wavelength, allowing stable control of light quantity within theguaranteed temperature range of the product.

As described above, the optical head apparatus of the present inventioncan implement anterior light monitoring with high-speed responsivity bycondensing a beam to a predetermined size on a photodetector, correct anastigmatic difference of a semiconductor laser using this lightreflection element and integrate the semiconductor laser andphotodetector in a single unit, thus making it possible to simplify andreduce the size of the optical head.

Furthermore, the configuration according to the present inventionmonitors the light quantity of laser radiating beams by effectivelyutilizing light beams outside the aperture, making it possible to reduceloss of light quantity, increase the monitoring light quantity byoptimizing the area and location of the reflection/diffraction grating,thus providing a high S/N ratio of monitor signals.

Furthermore, since the reflection type hologram itself provideshigh-level condensing, it is possible not only to reduce the size ofoptical spots on the photodetector but also reduce the optical detectionarea, making it possible to implement anterior light monitoring withhigh-speed responsivity and stabilize the recording quality by therecording type optical head such as DVD-RAM.

Furthermore, the present invention can simplify and reduce the size ofthe optical head by integrating the anterior light monitoringphotodetector, laser chip and signal detection photodetector, etc. in asingle unit.

What is claimed is:
 1. An optical head apparatus, comprising: a semiconductor laser light source; a photodetector that receives at least one part of light from said semiconductor laser light source; a light reflection element provided with; a peripheral section that reflects peripheral light of the light from said semiconductor laser light source and condenses it into said photodetector, a condensing function of the peripheral section of said light reflection element has an astigmatic difference; and a central section that transmits central light of the light from said semiconductor laser light source; and a condenser lens that condenses the light that passes through said light reflection element onto an optical disc, wherein: each surface of the central section of said light reflection element has a flat shape; and at least one surface of the peripheral section of said light reflection element has a spherical or non-spherical curved shape.
 2. The optical head apparatus according to claim 1, characterized in that the condensing function of the peripheral section of said light reflection element has a spherical aberration.
 3. An optical head apparatus, comprising: a semiconductor laser light source; a photodetector that receives at least one part of light from said semiconductor laser light source; a light reflection element provided with a peripheral section that reflects peripheral light of the light from said semiconductor laser light source and condenses it into said photodetector and a central section that transmits central light of the light from said semiconductor laser light source; and a condenser lens that condenses the light that passes through said light reflection element onto an optical disc, wherein: each surface of the central section of said light reflection element has a flat shape; at least one surface of the peripheral section of said light reflection element has a spherical or non-spherical curved shape, the spherical or non-spherical curved shape forming a surface of rotation about the central section of the light reflecting element; both surfaces of the central section of said light reflection element are parallel and inclined at a predetermined angle with respect to a direction perpendicular normal to an optical axis of the light from said semiconductor laser light source; and the astigmatism produced by the inclined placement of the central section of said light reflection element compensates the astigmatic difference of said semiconductor light source.
 4. The optical head apparatus according to claim 1, wherein: both surfaces of the central section of said light reflection element are parallel and inclined at a predetermined angle with respect to a direction perpendicular to an optical axis of the light from said semiconductor laser light source; and the astigmatism produced by the inclined placement of the central section of said light reflection element compensates the astigmatic difference of said semiconductor laser light source.
 5. The optical head apparatus according to claim 1, characterized in that one surface of the central section of said light reflection element is not parallel to the other surface.
 6. The optical head apparatus according to claim 1, wherein the optical axis of the reflected light from the peripheral section of said light reflection element is inclined with respect to the optical axis of the light from said semiconductor laser light source.
 7. The optical head apparatus according to claim 1, wherein said semiconductor laser light source and said photodetector are formed in one package.
 8. The optical head apparatus according to claim 1, wherein: a first surface of the peripheral section of said light reflection element is nearer the semiconductor laser light source than a second surface of the peripheral section of said light reflection element; and the first surface has a flat shape and the second surface has the spherical or non-spherical curved shape.
 9. An optical head apparatus, comprising: a semiconductor laser light source; a plurality of photodetectors placed adjacent to said semiconductor laser light source; a reflection type hologram element provided with a peripheral section that reflects and diffracts peripheral light of the light from said semiconductor laser light source and condenses it into one or said plurality of photodetectors and a central section that transmits central light of the light from said semiconductor laser light source; and a condenser lens that condenses the light that passes through the central section of said reflection type hologram element onto an optical disc, wherein: said photodetector that receives said reflected and diffracted light is placed closer, with respect to said semiconductor laser light source, in the direction of the major axis of an ellipse than in the direction of the minor axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source; and the photodetector that receives signal light from said optical disc is placed closer, with respect to said semiconductor laser light source, in the direction of the minor axis of the ellipse than in the direction of the major axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source.
 10. The optical head apparatus according to claim 9, wherein said reflection type hologram element reflects and diffracts more light in the direction of the major axis of the ellipse than light in the direction of the minor axis of the ellipse of the elliptic far field pattern of outgoing light from said semiconductor laser light source.
 11. The optical head apparatus according to claim 9, wherein the hologram formation area of said reflection type hologram element is formed more widely in the direction of the major axis of the ellipse with respect to the center of the axis of the elliptic far field pattern of said semiconductor laser.
 12. The optical head apparatus according to claim 10, wherein the hologram formation area of said reflection type hologram element is formed more widely in the direction of the major axis of the ellipse with respect to the center of the axis of the elliptic far field pattern of said semiconductor laser.
 13. The optical head apparatus according to claim 9, wherein the condensing function of the peripheral section of said reflection type hologram element has an astigmatic difference.
 14. The optical head apparatus according to claim 10, wherein the condensing function of the peripheral section of said reflection type hologram element has an astigmatic difference.
 15. The optical head apparatus according to claim 9, wherein the condensing function of the peripheral section of said reflection type hologram element has a spherical aberration.
 16. The optical head apparatus according to claim 10, wherein the condensing function of the peripheral section of said reflection type hologram element has a spherical aberration.
 17. The optical head apparatus according to claim 9, wherein: both surfaces of the central section of said reflection type hologram element are parallel; these surfaces are inclined at a predetermined angle with respect to the direction perpendicular to the optical axis of the light from said semiconductor laser light source; and the astigmatism produced by the inclined placement of the central section of said reflection type hologram element compensates the astigmatic difference of said semiconductor laser light source.
 18. The optical head apparatus according to claim 10, wherein: both surfaces of the central section of said reflection type hologram element are parallel; these surfaces are inclined at a predetermined angle with respect to the direction perpendicular to the optical axis of the light from said semiconductor laser light source; and the astigmatism produced by the inclined placement of the central section of said reflection type hologram element compensates the astigmatic difference of said semiconductor laser light source.
 19. The optical head apparatus according to claim 9, wherein one plane of the central section of said reflection type hologram element is not parallel to the other plane.
 20. The optical head apparatus according to any one of claim 10, wherein one plane of the central section of said reflection type hologram element is not parallel to the other plane.
 21. The optical head apparatus according to claim 9, wherein the optical axis of reflected and diffracted light from the peripheral section of said reflection type hologram element is inclined with respect to the optical axis of light from said semiconductor laser light source.
 22. The optical head apparatus according to claim 10, wherein the optical axis of reflected and diffracted light from the peripheral section of said reflection type hologram element is inclined with respect to the optical axis of light from said semiconductor laser light source.
 23. The optical head apparatus according to claim 9, wherein said semiconductor laser light source and a plurality of photodetectors provided adjacent thereto are formed in one package.
 24. The optical head apparatus according to claim 9, further comprising a polarized hologram element that allows light from said laser light source to penetrate and the light reflected by said optical disc to diffract, wherein: said polarized hologram element is mounted on a movable part of an objective lens actuator together with an objective lens; and the light transmission area of the central section of said reflection type hologram element has a quasi-elliptic shape whose major axis lies in the direction of the tracking operation of said objective lens actuator.
 25. The optical head apparatus according to claim 9, further comprising a polarized hologram element that allows light from said laser light source to penetrate and the light reflected by said optical disc to diffract, wherein said polarized hologram element is integrated with said reflection type hologram element.
 26. The optical head apparatus according to claim 9, wherein: the light condensing point by said reflection type hologram element is apart from the plane of a photoreception element of said photodetector at a room temperature; the light condensing point moves by wavelength fluctuations due to temperature variations; and said light condensing point aligns with the plane of said photoreception element in the vicinity of a mid point of the operating temperature range of said optical head apparatus.
 27. The optical head apparatus according to claim 9, wherein the hologram formation area of said reflection type hologram element is formed asymmetric with respect to a point centered on the optical axis of the light from said semiconductor laser light source.
 28. An optical head apparatus, comprising: a semiconductor laser light source; a photodetector that receives at least one part of light from said semiconductor laser light source; a light reflection element provided with a peripheral section that reflects peripheral light of the light from said semiconductor laser light source and condenses it into said photodetector and a central section that transmits central light of the light from said semiconductor laser light source; and a condenser lens that condenses the light that passes through said light reflection element onto an optical disc, wherein: both surfaces of the central section of said light reflection element have a flat shape and are parallel; normals of both surfaces of the central section of said light reflection element are inclined at a predetermined angle with respect to a direction of an optical axis of the light from said semiconductor laser light source; and astigmatism produced by the inclined placement of the central section of said light reflection element compensates an astigmatism difference of said semiconductor laser light source. 